The present invention relates to a semiconductor laser and a production method thereof, and particularly to a self pulsation type semiconductor laser and a production method thereof.
FIG. 16 is a schematic sectional view, seen along the direction perpendicular to the resonator length direction, of a related art inner stripe type semiconductor laser.
Layers are epitaxially grown in sequence on the entire surface of a substrate 1 made from n-type GaAs: a first cladding layer 2 made from n-type Al0.5Ga0.5As, an active layer 3 made from Al0.15Ga0.85As, a second cladding layer 4 made from p-type Al0.5Ga0.5As, and a heavily doped contact layer 5 made from p-type GaAs. The stacked layers are selectively etched from the contact layer 5 side up to a depth reaching the inner portion of the second cladding layer 4, to form two grooves 8, thereby forming a stripe-like ridge 7 extending in the direction perpendicular to the paper plane of FIG. 16 between the grooves 8. In this case, the depth of the groove 8 is selected such that the second cladding layer 4 having a specific thickness xe2x80x9cdxe2x80x9d remains under the grooves 8.
A current constriction layer 6 made from n-type GaAs is grown in such a manner as to bury the grooves 8.
A first electrode 9 is formed on the contact layer 5 and the current constriction layer 6 in such a manner as to be in ohmic-contact therewith, and a second electrode 10 is formed on the back surface of the substrate 1 in such a manner as to be in ohmic-contact therewith.
In the semiconductor laser having such a configuration, the active layer 3 is divided into a gain region 11, two saturable absorption regions 12 on both sides of the gain region 11, and two outside regions 13 on both sides of the saturable absorption regions 12.
A current, which is restrictively supplied to the stripe-like ridge 7 by the effect of the current constriction layer 6, is injected in the gain region 11 of the active layer, with a result that a gain necessary for laser oscillation occurs only in the gain region 11 of the active layer 3.
The saturable absorption region 12 does not undergo current injection, and acts as a light saturable absorber which does not absorb light when the light intensity increases to some extent and becomes a transparent body.
The saturable absorber, therefore, acts as a Q switch, which is capable of adjusting the ratio of light effused from the gain region 11 to the saturable absorption region 12 by selecting the width xe2x80x9cWxe2x80x9d of the gain region 11 and the thickness xe2x80x9cdxe2x80x9d of each of portions, on both sides of the stripe-like ridge 7, of the second cladding layer 4. The output of the laser light is periodically changed by adjusting the ratio of light effused from the gain region 11 to the saturable absorption region 12, to thus constitute a self pulsation type semiconductor laser.
A light distribution region upon operation is schematically shown by a chain line xe2x80x9caxe2x80x9d in FIG. 16.
Such a self pulsation laser, which is low in coherence of laser light and also low in a so-called optical feedback induced noise due to an unstable laser oscillation state caused by return of light, having been emitted from the laser, to the laser again, is useful as an optical disk light source or a high-speed LAN (Local Area Network) light source.
It is experimentally known that the above-described self pulsation laser is obtained by selecting the width xe2x80x9cWxe2x80x9d of the gain region 11 at a narrow value (generally, 5 xcexcm or less), and setting a difference xcex94n in effective refractive index between the gain region 11 and the saturable absorption region 12 at a small value (generally, xcex94nxe2x89xa60.01) by adjusting the thickness xe2x80x9cdxe2x80x9d of each of the portions, on both the sides of the stripe-like ridge 7, of the second cladding layer 4. However, since the allowable ranges of the width xe2x80x9cWxe2x80x9d and the thickness xe2x80x9cdxe2x80x9d are narrow, it is difficult to adjust the width xe2x80x9cWxe2x80x9d and the thickness xe2x80x9cdxe2x80x9d at the etching step for forming the grooves 8.
Accordingly, it is difficult to sustain the self pulsation at a high light output, for example, 10 mW, and also it is difficult to sustain the self pulsation at a high operational temperature, for example, 70xc2x0 C. Further, it is difficult to produce a self pulsation laser with a high production yield.
An object of the present invention is to provide a self pulsation type semiconductor laser capable of sustaining the self pulsation at a high light output and/or at a high operational temperature, and to provide a method of producing the semiconductor laser.
Another object of the present invention is to provide a self pulsation type semiconductor laser which is produceable with a high production yield, and to provide a method of producing the semiconductor laser.
A semiconductor laser according to the present invention basically includes a first cladding layer, an active layer, a second cladding layer, and a current constriction layer.
The active layer may be formed in such a manner that it has a gain region which is defined as a current injection region by the current constriction means and which is capable of acquiring an optical gain by current injection thereto; a saturable absorption region in which there occurs light effusion thereto; and an outside region in which there little occurs light effusion thereto. The active layer may be also formed in such a manner that it has only a gain region, and a saturable absorption layer, which has a saturable absorption region disposed at such a position as to allow the region to absorb light from the gain region and also has an outside region disposed outside the saturable absorption region in such a manner as to be in contact therewith, is provided, separately from the active layer, in at least one of the first and second cladding layers.
In each of these configurations, an effective band gap of the saturable absorption region may be larger than that of the outside region.
A method of producing a semiconductor laser according to the present invention basically includes steps of sequentially growing a first cladding layer, an active layer, and a second cladding layer on a substrate, and forming a current constriction means.
The current constriction means can be formed in accordance with a related art method.
In this method, the active layer may be formed in such a manner that it has a gain region which is defined as a current injection region by the current constriction means and which is capable of acquiring an optical gain by current injection thereto; a saturable absorption region in which there occurs light effusion thereto; and an outside region in which there little occurs light effusion thereto. The active layer may also be formed in such a manner that it has only a gain region, and a saturable absorption layer, having a saturable absorption region, disposed at such a position as to allow the region to absorb light from the gain region, in which there occurs light effusion thereto, and also having an outside region, being in contact with the saturable absorption region, in which there little occurs light effusion thereto, is provided separately from the active layer.
In each of these methods, an effective band gap of the saturable absorption region may be larger than that of the outside region.
According to the semiconductor laser of the present invention having the above-described configuration, it is possible to sustain the self pulsation at a high light output and/or a high operational temperature.
With respect to a self pulsation type semiconductor laser, it is known that the function of a saturable absorption region can be made higher by making the carrier lifetime in the saturable absorption region shorter than the carrier lifetime in a gain region and/or by making the differential gain in the saturable absorption region larger than the differential gain in the gain region (see H. Kawaguchi, Appl. Phys. Lett., 45(12)pp. 1264 (1984); M. Ueno and R. Lang, J. Appl. Phys., 58(4)pp. 1689 (1985); and H. Adachi, S. Kaminoyama, I. Kidoguchi, and T. Uenoyama, IEEE Photon. Technol. Lett., 7(12)pp. 1406 (1995)).
By making higher the function of the saturable absorption region as described above, it is possible to sustain the self pulsation at a higher light output and/or a higher operational temperature.
On the other hand, while one factor of defining the carrier lifetime in a saturable absorption region is physical properties of a semiconductor crystal forming the saturable absorption region, the present inventor has found the fact that, in the case where carriers can be migrated from the saturable absorption region to the outside region being in contact therewith, the migration of the carriers becomes a factor of defining an effective carrier lifetime in the saturable absorption region.
To be more specific, if carriers are readily migrated from the saturable absorption region to the outside region, the carrier lifetime in the saturable absorption region is actually shorter than the carrier lifetime defined by the physical properties of the semiconductor crystal forming the saturable absorption region. On the contrary, if the carriers are not migrated or slowly migrated from the saturable absorption region to the outside region, the carrier lifetime in the saturable absorption region is actually equal to or only slightly shorter than the carrier lifetime defined by the physical properties of the semiconductor crystal forming the saturable absorption region.
The semiconductor laser of the present invention has been made on the basis of the above-described knowledge, and is characterized in that an effective band gap of a saturable absorption region is set to be larger than that of an outside region. With this configuration, because of a property of carriers easy to be migrated from a region having a large effective band gap to a region having a small effective band gap, carriers generated in the saturable absorption region due to optical absorption are readily migrated to the outside region having a small effective band gap, so that the carrier lifetime in the saturable absorption region is actually shortened, to thereby enhance the function of the saturable absorption region.
The semiconductor laser of the present invention thus obtained can sustain the self pulsation at a high light output and/or a high operational temperature.